For the sealing of kefir containers the use of printed aluminum foil, the edges of which are crimped and sealed around the opening of the container, is known. Since kefir is effervescent the seal should not be completely tight, since in that case pressure can build up in the container, thereby lifting the foil or destroying it. For this reason the foil is perforated with needles after filling, so that the gas can escape.
This method, however, has not proved to be practical, since spore-carrying dust and germs can enter the container and spoil the contents. It is well known that milk products are very susceptible to infection and that even the smallest amounts of dirt can lead to the formation of mold.
Moreover, the perforated openings in the aluminum foil often are blocked by the milk serum, so that pressure which builds up in the container cannot be relieved. Because of the vibration during the automatic filling of the containers on a conveyor, splashes which adhere to the underside of the lid cause the openings to be covered by plugs formed when the sugar-containing milk serum thickens.